The key here is good strategy. I used Atsuma, Raigar, Karin and Yuki. It has been suggested you can use Ruby Butterfly as well as a healer, but Karin has Hi-Return Dance which comes in handy when an ally falls. Stock up on healing items at the shop in the village first. The battle starts with Oboro and 3 ninjas. Move everyone to the back row. This is essential; if you try to go toe to toe with him you will get pounded unless you're at a high level. My party was in their mid-thirties for this battle. Try to use everyone's EX abilities; World Creation and Mega Phoenix work well. Once the three ninjas are dead, Oboro summons two clones. Kill one and leave one; otherwise he will just summon more. If you stay in the back row and heal/revive as needed, Oboro should fall in no time.